I've read Gravity's Rainbow, V, Vineland, Crying of Lot 49, Bleeding Edge and Inherent Vice... I think that's it. I also started Mason and Dixon and was enjoying it a lot until I left it on a bus or something.
I would rank as follows, from worst to best....
Bleeding Edge - what the fuck was he thinking?
Inherent Vice - bit of fun but nothing more, verges on kooky or even wacky at times, that's not a good thing
V - huge jump in quality from the previous two here - but book is dense and nerdy without quite enough vim or even brio to elevate it off the ground
Vineland - I think maybe the first one I read and I enjoyed it a lot, it was all new to me; exciting, funny, weird, clever, thanates
Crying of Lot 49 - really good book that somehow pulls off the impossible trick of packing in as many ideas as his other books despite being only 4 pages long. Paranoia, conspiracies, twisty-turny stuff, that hilarious play called The Courier's Tragedy or something which at the time almost made me cry with laughter, a great mysterious logo - I don't really remember this book well but I remember enjoying it a lot
Gravity's Rainbow - big book with big ideas but also loads and loads of little ones, countless locations and digressions that are worth digressing to (the white lodge or mansion or whatever, the lightbulb that lasts forever etc), world war two, madness - the only slight disappointment for me is the way that Slothrop fails to really convince as anything - I dunno if he's supposed to be merely a cipher - an axle the events revolve around - or what but I feel that cos he turns up in some of the other books Pynchon maybe thinks that he has created a more interesting character than he actually has - though strangely, cos of the unusual nature and structure of the novel and its sheer scope it doesn't matter half as much as you might think... and also there is a badly misjudged scene about weird sweets which is supposed to be funny but more made me cringe. That doesn't sound important perhaps but it comes near the start and it almost put me off cos I think sense of humour is way of telling if you're on the same wavelength.
Mason and Dixon, I had only just started and I guess it had like 850 pages in which to go wrong, but from the start it was looking very promising. A shame I lost it, at that point in my life I seemed to lose a lot of books... almost every time I went on a bus or sat down on a bench or whatever I seemed to be halfway through the journey and think "hang on a minute, wasn't i reading a book a minute ago?". Very disappointing cos I normally buy books from charity shops (not always but I find it a nice thing to do; you never know what you'll get, there's a really interesting and random choice and they are unbelievably cheap, you can walk out with an artwork... a masterpiece maybe and it costs less than a can of coke, it's crazy) and if you do that you can never really rely on easily replacing something if you lose it or destroy it or whatever, unless it's 50 Shades of Grey or Wolf Hall or Twilight or something by Bill Bryson... well, there are quite a lot of things that are in every single one but that doesn't matter cos there are loads of things that just pop up and make them interesting. But I digress. I should get back to Mason and Dixon some time, or perhaps I've read enough of this guy.